ability to fetch dependencies via git+ssh protocol

[andrewrk]

Extracted from #14265.

zig build should support fetching via an URL like this:

    .url = "git+ssh://git@github.com:ziglang/zig.git#8b8090d7fad3e444784bc52db6a80188a9dbd3c0",

Note that the fragment is used to fetch a particular commit. I suppose the fragment could be omitted, meaning to fetch the latest HEAD of the default branch, however, this would be not advised since the hash would be wrong as soon as another commit is pushed to that branch. Ideally, if the fragment is omitted then an error would be emitted telling the user to add the fragment, giving them a copy+pasteable snippet, or perhaps even editing the manifest file on the user's behalf.

Open question: should it be built-in? or is this issue a request for a third party contributor to make a fetch plugin (#14294)?

I think the first step would be to implement this as a third-party fetch plugin, and then we can evaluate whether it can be upstreamed and become a builtin.

Related:

• ability to fetch dependencies via subversion protocol #14296
• ability to fetch dependencies via mercurial protocol #14297
• ability to fetch dependencies via git+http and git+https protocol #14298